The present invention relates to induction heated servers for heating and/or maintaining the temperature of food in serving plates placed thereon.
Food must be served at temperatures above 140° F. to maintain good taste and quality. This is a challenge in food service applications where meals are delivered to remote locations such as in hospitals and nursing homes. Meals are served on china plates or serving dishes in a central kitchen and then transported to patients in their rooms. To keep the food warm, the served food is placed on a special heated “base” or server and then covered with an insulated dome. Typically, the food temperature must be held above 140° F. for up to 1 hour.
The special base or server is itself heated by an electromagnetic induction process. The base assembly includes an encapsulated “load” or heat retentive disc which is (1) susceptible to electromagnetic excitation and converts electromagnetic energy into equivalent heat energy, and (2) desirably capable of storing both latent and sensible heat energy at temperatures in the range of 200° F. through 350° F. Prior to placing the food plate on the special base or server, the base is electromagnetically charged with a typical 60,000 joules (57 BTU) of energy. Typically, the charging power is 3,300 watts which is maintained over a typical charging time of 18 seconds. As the base is inductively heated, the load temperature increases to as high as 350° F. In the process the load stores latent heat energy within a relatively narrow temperature range, e.g., between 250° and 350° F.
The serving plate with the food thereon is placed on the server and the latent energy stored in the load is slowly transferred to the food by thermal conduction to maintain the food at an elevated temperature above 140° F. The driving force for the heat transfer is the difference in temperature between the load temperature and the food temperature.
Immediately after charging the server, the food is typically at a served temperature of 180° F., and the load may reach a maximum post-charging internal temperature as high as 350° F. At the end of 1 hour, the food temperature and load temperature fall to about 140° F. Thus the rate of heat transfer into the food zone is greatest within a few minutes after charging the base when the temperature difference between the load and the food is greatest. As heat is gradually transferred to the ambient environment through the base and dome cover, the latent energy in the load becomes depleted and the load temperature begins to fall along with the food temperature.
Current server technology is exhibited by the serving system sold by Dinex International Corporation under the designation Model 511 Smart Therm™. The server or base assembly consists of two molded synthetic resin elements—upper and lower. The load and insulation layer are sandwiched between the upper and lower elements which are ultrasonically welded to each other. The insulation on the bottom of the load substantially prevents heat loss through the bottom element and promotes heat flow from the load into the upper surface of the upper element. It is important to have intimate contact between the load and the inner surface of the upper element. The intimate contact promotes good thermal conduction between the load and the food zone above the upper plastic element. A radio frequency identification (RFID) tag can be incorporated to provide information to the induction charger to prevent overheating of the temperature of the server.
Aladdin employs technology in which the load is an inductively susceptible metal plate. A disadvantage of the Aladdin technology is that the thermal storage in the metal plate is 100 percent sensible heat and 0 percent latent storage. The Aladdin system is not capable of storing energy to the same level as the Dinex server without using a heavy and costly metal load that operates at temperatures in excess of 500° F. Because of the high temperature, air trapped inside the Aladdin server expands and creates a high internal pressure. High pressure will also be developed if water penetrates into the induction zone (when the server is washed) through a faulty perimeter seal. A pressure relief valve is provided to vent the high pressure air or moisture that, if not otherwise relieved, could create an unsafe condition. The reliability of the Aladdin system is diminished by failure of the pressure relief valve which itself can result in water infiltration during the washing process. The ability of the base to store heat at high temperature is greatly diminished once water enters into the induction zone because the water acts to remove heat and reduce temperature as heated steam escapes through the relief valve.
The 511 Dinex base design is inherently less sensitive to the effects of trapped air. Pressure increase is lessened because the Dinex latent heat load does not require very high temperatures as are required with the Aladdin metal plate load. Thus, a pressure relief valve is not required in the Dinex server.
However, the reliability of this Dinex server is compromised by the need to ultrasonically weld the upper and lower plastic elements together. Experience has shown that the weld seam is not reliable, and water can infiltrate the induction zone during washing when the seal is not 100 percent hermetic. After inductive charging, steam escapes through any perimeter leak, thereby venting heat to the atmosphere. When this happens, the server loses its ability to keep the food warm.
It is an object of the present invention to provide a novel server construction which effectively precludes water infiltration into the server.
It is also an object to provide such a server which eliminates the need for welding the two elements.
Another object is to eliminate or minimized trapped air inside the server to preclude the creation of high internal pressure and resultant mechanical deformation of the server when inductively heated.
Another object of the present invention is to provide such server that reduces heat losses to the atmosphere and maximize heat transfer into the food zone.
Another object is to provide such a server which is readily fabricated and long lived.
A further object is to provide a novel method for fabricating an improved server which eliminates the need for welding top and bottom elements.